PD-DM: An efficient locality-preserving block device mapper with plausible deniability

Abstract

Encryption protects sensitive data from unauthorized access, yet is not sufficient when users are forced to surrender keys under duress. In contrast, plausible deniability enables users to not only encrypt data but also deny its existence when challenged. Most existing plausible deniability work (e.g. the successful and unfortunately now-defunct TrueCrypt) tackles “single snapshot” adversaries, and cannot handle the more realistic scenario of adversaries gaining access to a device at multiple time points. Such “multi-snapshot” adversaries can simply observe modifications between snapshots and detect the existence of hidden data. Existing ideas handling “multi-snapshot” scenarios feature prohibitive overheads when deployed on practically-sized disks. This is mostly due to a lack of data locality inherent in certain standard access-randomization mechanisms, one of the building blocks used to ensure plausible deniability. In this work, we show that such randomization is not necessary for strong plausible deniability. Instead, it can be replaced by a canonical form that permits most of writes to be done sequentially. This has two key advantages: 1) it reduces the impact of seek due to random accesses; 2) it reduces the overall number of physical blocks that need to be written for each logical write. As a result, PD-DM increases I/O throughput by orders of magnitude (10–100× in typical setups) over existing work while maintaining strong plausible deniability against multi-snapshot adversaries. Notably, PD-DM is the first plausible-deniable system getting within reach of the performance of standard encrypted volumes (dm-crypt) for random I/O.

SqORAM: Read-Optimized Sequential Write-Only Oblivious RAM

“…Plausible deniability ultimately aims to enable users to deny the very existence of sensitive information on storage media when confronted by coercive adversaries e.g., border officers in oppressive regimes. This is essential in the fight against increasing censorship and intrusion into personal privacy [7,20] Unfortunately, it is impractical to deploy existing ORAM mechanisms in such systems due to prohibitively-high access latencies deriving from high asymptotic overheads for accessing items and ORAMinherent randomized access patterns. Also, a full ORAM protocol protecting access patterns of all operations in real time may be unnecessary for plausible-deniability.…”

“…For example, HIVE is almost four orders of magnitude slower than HDDs and two orders of magnitude slower than SSDs. The main contributor to this slowdown is the random placement of data meant to break linkability between separate writes [7,24], an important property ensuring that an adversary cannot link a set of writes to each other logically, given multiple snapshots of the media. Random data placement results in dramatically increased disk-seek related latencies.…”

INFUSE: Invisible plausibly-deniable file system for NAND flash

“…In practice, protecting against such powerful adversaries is essential due to increasing instances of intrusion by unfriendly powerful nation state adversaries [15,21]. This is sometimes a matter of life and death [23] as documented in numerous cases where information had to be transferred through checkpoints manned by hostile adversaries.…”

“…To protect against multi-snapshot adversaries, several recent systems [12,14,15,23] have proposed mechanisms that plausibly "explain" all device state changes via public data operations, in effect protecting access patterns to hidden data.…”

“…Yet, all existing PDS feature a critical shortcoming that renders them insecure in practice and may subject their users to "rubberhose" attacks: the system is unable to hide the fact that a special storage mechanism is being used which potentially allows the user to hide data. In particular, the presence of system-specific metadata e.g, special indexing structures stored on disk to track hidden information [12,14,15] or other design artifacts such as non-standard on-disk data layout [23], ultimately reveal to the adversary that the user is using a plausible deniable storage system. However, the use of a PDS can in itself raise suspicion -since why would one use such a storage system especially if it is slower than existing standard systems -unless there is something to be hidden.…”